Differential Diagnosis

• Single most likely diagnosis
  • Serum sickness-like reaction (SSLR): This condition is a likely diagnosis given the patient's recent history of amoxicillin use, which is a common cause of SSLR. The patient's symptoms of arthralgias, rash, and swelling are consistent with this diagnosis. The rash appeared after the patient finished the antibiotic course, which is also consistent with SSLR.
• Other Likely diagnoses
  • Crystal arthropathy (gout or pseudogout): The patient's symptoms of joint pain and swelling, particularly in the ankles and knees, could be consistent with crystal arthropathy. However, the presence of a rash and the recent antibiotic use make this diagnosis less likely.
  • Septic arthritis: Although the patient had a tooth infection and was on antibiotics, the lack of systemic symptoms such as fever and the absence of tingling or numbness make this diagnosis less likely.
  • Psoriatic arthritis: The patient's rash and joint symptoms could be consistent with psoriatic arthritis, but the absence of other symptoms such as nail changes or axial involvement make this diagnosis less likely.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
  • Lyme disease: Although the patient does not have a history of tick exposure, Lyme disease can cause joint pain and swelling, as well as a rash. Missing this diagnosis could lead to serious consequences, such as cardiac or neurological involvement.
  • Infectious endocarditis: The patient's recent history of tooth infection and antibiotic use could be a risk factor for infectious endocarditis. Although the patient does not have systemic symptoms, this diagnosis should be considered due to its potential severity.
• Rare diagnoses
  • Erythema nodosum: This condition is a type of skin inflammation that can cause red bumps and joint pain. Although it is a rare diagnosis, it could be considered given the patient's symptoms.
  • Henoch-Schönlein purpura: This condition is a type of vasculitis that can cause joint pain, rash, and gastrointestinal symptoms. Although it is a rare diagnosis, it could be considered given the patient's symptoms.